This invention relates to pressure sensitive adhesives formed using gamma irradiation, methods of using the pressure sensitive adhesives, and articles containing the pressure sensitive adhesives.
The advantage of acrylic polymers as viscoelastic bases for pressure-sensitive adhesives (PSAs) are well known. Pressure-sensitive adhesives have been used for more than half a century for a variety of marking, holding, protecting, sealing, and masking purposes. Fundamentally, PSAs require a delicate balance of viscous and elastic properties, which result in a 4-fold balance of adhesion, cohesion, stretchiness, and elasticity. In essence, PSA products have sufficient cohesiveness and elasticity so that, despite their tackiness, they can be handled with the fingers and can typically be removed from smooth surfaces without leaving a substantial amount of residue.
There are several methods presently in use for the preparation of PSAs. These methods include a variety of polymerization methods including batch, hot melt, solution, thermal emulsion, suspension, ultra-violet (UV)-initiated bulk, and UV-initiated on-web polymerization techniques. Monomer and initiator residues produced in some of these methods can prevent the attainment of desirable levels of properties (e.g., peel adhesion and shear strength) and some of these methods are relatively slow.
For example, acrylic polymer compositions can be used to make PSAs by solution polymerization. The solution polymerization methods generally use relatively large amounts of organic solvents. Polymers in solvent may be difficult to handle and transport due to the volume of the solvent and the potential release of volatile organic compounds (VOCs) into the atmosphere. Using solvents also necessitates high heat or vacuum to remove the solvent from the polymer.
Generally, the present invention relates to pressure sensitive adhesives formed using gamma irradiation. One embodiment is a pressure sensitive adhesive comprising a polymeric reaction product formed by gamma ray irradiation of an emulsion composition. The emulsion composition comprises water; (meth)acrylate monomer material; polar, free-radically polymerizable material that is copolymerizable with the (meth)acrylate monomer material; and at least one emulsifier. Yet another embodiment is like the first embodiment, as described above, wherein the polymeric reaction product is formed in the absence of any initiator selected from the group consisting of photoinitiators and thermal initiators.
Another embodiment is a pressure sensitive adhesive-forming emulsion composition comprising a polymeric reaction product formed by gamma irradiation of a composition. The composition comprising water; (meth)acrylate monomer material; polar, free-radically polymerizable material that is copolymerizable with the (meth)acrylate monomer material; and at least one emulsifier.
Yet another embodiment is a method of making a pressure sensitive adhesive, the method comprising the steps of: forming an emulsion composition comprising water, (meth)acrylate monomer material, polar, free-radically polymerizable material that is copolymerizable with the (meth)acrylate monomer material, and at least one emulsifier; and irradiating at least a portion of the emulsion composition with gamma rays to initiate polymerization of the emulsion composition. An additional embodiment is the method described above and further comprising the step of removing at least a portion of the water from the emulsion composition after irradiating at least a portion of the emulsion composition. A further embodiment is the method, as described above, wherein the step of irradiating at least a portion of the emulsion composition comprises irradiating the emulsion composition substantially uniformly.
An additional embodiment is an article comprising: a substrate; and a pressure sensitive adhesive disposed on at least one surface of the substrate, the pressure sensitive adhesive comprising a polymeric reaction product formed by gamma irradiation of an emulsion composition comprising water, (meth)acrylate monomer material, polar, free-radically polymerizable material that is copolymerizable with the (meth)acrylate monomer material, and at least one emulsifier.
Another embodiment is a pressure sensitive adhesive which is a polymeric reaction product of a composition. The composition comprises: water; (meth)acrylate monomer material; polar, free-radically polymerizable material that is copolymerizable with the (meth)acrylate monomer material; and an emulsifier. The polymeric reaction product has a shear strength of at least 5000 minutes at room temperature using a 1 kg weight and a peel adhesion of at least 45 N/dm at, at least room temperature. In another embodiment, the shear strength was at least 10,000 minutes. In yet another embodiment, the peel adhesion was at least 50 N/dm. In another embodiment, the peel adhesion was at least 50 N/dm and the shear strength was at least 10,000 minutes.
The above summary of the present invention is not intended to describe each disclosed embodiment or every implementation of the present invention. The detailed description which follows more particularly exemplifies these embodiments.
The present invention is believed to be applicable to pressure sensitive adhesives, methods of using the pressure sensitive adhesives, and articles containing the pressure sensitive adhesives. In particular, the present invention is directed to pressure sensitive adhesives formed by gamma irradiation, methods of using the pressure sensitive adhesives, and articles containing the pressure sensitive adhesives. While the present invention is not so limited, an appreciation of various aspects of the invention will be gained through a discussion of the examples provided below.
At least some embodiments of both the pressure sensitive adhesives of the invention and the methods of the invention to make pressure sensitive adhesives offer advantages not available currently. Water-based latex pressure sensitive adhesives of the invention can have excellent adhesive properties including a satisfactory range of both peel adhesion and shear strength performance. Some embodiments are particularly useful in applications that are adversely impacted if initiator residue is present in the adhesive. The method of making pressure sensitive adhesives can enable the use of higher solids emulsions. This can result in less need to adjust emulsion viscosity for a subsequent coating operation and lower energy cost to dry the applied adhesive coating.
Generally, the pressure sensitive adhesives of the invention are formed by gamma radiation polymerization of an emulsion containing water; (meth)acrylate monomer material; polar, free-radically copolymerizable material that is copolymerizable with the (meth)acrylate monomer material; and an emulsifier. Other components can also be added for particular application or to obtain desired properties including, for example, other copolymerizable materials, tackifying agents, crosslinking agents, free radical initiators, plasticizers, dyes, pigments, antioxidants, UV stabilizers, thickening agents, electro-conductivity agents, reflective agents, antistatic agents, inorganic materials, biocides (bactericides, fungicides), bioactive agents, pharmaceutical aids, releasable agents, cosmetic agents, rheology modifiers, and the like. The pressure sensitive adhesives can, if desired, have good peel adhesion, good shear strength, or a combination of these properties.
The terms xe2x80x9c(meth)acrylatexe2x80x9d or xe2x80x9c(meth)acrylatesxe2x80x9d are used throughout this application and are meant to include both acrylate(s) and methacrylate(s).
The amounts of each component in the emulsion are typically present in a number of parts per 100 parts of the combined (meth)acrylate monomer material; polar, free-radically copolymerizable material; and any other copolymerizable material. Henceforth this basis is called the total free-radically copolymerizable material.
Free-radically polymerizable (meth)acrylate monomer material is employed in the emulsion to make the pressure sensitive adhesives. The (meth)acrylate monomer material includes at least one type of free-radically polymerizable acrylate monomer. In some embodiments, two or more different (meth)acrylate monomers are used. (Meth)acrylate monomer materials are generally esters of acrylic acid. Suitable (meth)acrylate monomers typically have only one free-radically polymerizable group. These compounds, when homopolymerized, generally have a glass transition temperature of no more than about 10xc2x0 C., preferably no more than about 0xc2x0 C. and, more preferably, no more than about xe2x88x9210xc2x0 C.
Examples of suitable (meth)acrylate monomers include acrylate esters of non-tertiary alkyl alcohols, the alkyl groups of which have from about 3 to about 13 carbon atoms. Examples of such (meth)acrylate monomers include, but are not limited to, isooctyl acrylate, 2-ethylhexyl acrylate, 4-methyl-2-pentyl acrylate, 2-methylbutyl acrylate, isoamyl acrylate, sec-butyl acrylate, n-butyl acrylate, tert-butyl acrylate, isobornyl acrylate, dodecyl acrylate, n-octyl acrylate, tridecyl acrylate, cyclohexyl acrylate, ethoxylated nonyl phenyl acrylate, methyl methacrylate, t-butyl methacrylate, iso-butyl methacrylate, butyl methacrylate, cyclohexyl methyl acrylate, hexyl methacrylate, iso decyl methacrylate, and hexyl ethyl methacrylate.
The polar, free-radically copolymerizable material useful in the emulsions to form pressure sensitive adhesives can include monomers, oligomers, and macromonomers. These materials generally have only one functional group which readily copolymerizes with the (meth)acrylate monomer material. The polar, free-radically copolymerizable material can be formed using a single compound or two or more compounds, as desired.
The polar, free-radically copolymerizable material generally provides hydrogen bonding to affect the properties of the pressure sensitive adhesive, such as, for example, increasing the cohesive strength of the resulting polymer. The polar, free-radically copolymerizable material is typically selected from the group of materials including ethylenically unsaturated carboxylic, sulfonic, and phosphonic acids (and their salts); ethylenically unsaturated anhydrides; ethylenically unsaturated amines and amides; N-vinyl lactams; ethylenically unsaturated alcohols; ethylenically unsaturated nitriles; and ethylenically unsaturated polyethers and polyesters.
Suitable polar, free-radically copolymerizable materials include, but are not limited to, acrylic acid, methacrylic acid, itaconic acid, crotonic acid, maleic acid, fumaric acid, vinyl phosphonic acid, 2-acrylamido-2-methylpropylsulfonic acid, maleic anhydride, N,N-dimethylaminoethylacrylate, N,N-dimethylaminoethylmethacrylate, N-vinyl pyrrolidone, N-vinyl caprolactam, acrylamide, t-butyl acrylamide, N,N-dimethyl amino ethyl acrylamide, N-octyl acrylamide and other N-substituted acrylamides, N,N-dimethylacrylamide and other N,N-disubstituted acrylamides, 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, 3-hydroxypropyl acrylate, 3-hydroxypropyl methacrylates, acrylonitrile, methacrylonitrile, carbowax acrylate, methoxy-ethoxy-ethyl acrylate, mixtures thereof, and the like. Preferred monomers include acrylic acid, methacrylic acid, N-vinyl pyrrolidone, hydroxyethyl acrylate, acrylamide, and mixtures thereof
Generally, the (meth)acrylate monomer material is provided in the emulsion in an amount that is sufficiently low to result in a stable emulsion and sufficiently high to improve the shear properties of the pressure sensitive adhesive. Typically, the (meth)acrylate monomer is provided in an amount ranging from about 84 to 98 parts by weight and the polar, free-radically copolymerizable material is provided in an amount ranging from about 2 to 6 parts by weight.
The (meth)acrylate monomer material or polar, free-radically copolymerizable material can contain two or more different components (e.g., the (meth)acrylate component may comprise two or more different (meth)acrylates that can be combined in any ratio) as long as the glass transition temperature of the total polymerizable material is no more than about 10xc2x0 C.
In addition to the (meth)acrylate monomer material and the polar, free-radically copolymerizable material, the emulsion composition may contain other copolymerizable materials. Typically, these materials are used in place of some of the (meth)acrylate monomer material and are copolymerizable with the (meth)acrylate monomer material.
Vinyl ester monomers are generally suitable for use as a copolymerizable material in the emulsions (to enhance cohesive strength). Examples of suitable vinyl ester monomers include unsaturated vinyl esters of linear or branched carboxylic acids having 1 to 12 carbon atoms. Such vinyl ester monomers include, but are not limited to, vinyl 2-ethylhexanoate, vinyl caprate, vinyl laurate, vinyl pelargonate, vinyl hexanoate, vinyl propionate, vinyl decanoate, and vinyl octanoate. Preferred vinyl ester monomers include vinyl acetate, vinyl laurate, vinyl caprate, vinyl-2-ethylhexanoate, styrene and mixtures thereof.
Suitable copolymerizable oligomers and macromonomers include acrylate-terminated poly(methyl methacrylate), methacrylate-terminated poly(methyl methacrylate), p-vinyl benzyl-terminated poly(methyl methacrylate), acrylate-terminated poly(styrene), methacrylate terminated poly(styrene), acrylate-terminated poly(ethylene oxide), methacrylate-terminated poly(ethylene oxide), acrylate-terminated poly(ethylene glycol), methacrylate-terminated poly(ethylene glycol), methoxy poly(ethylene glycol) methacrylate, butoxy poly(ethylene glycol) methacrylate, p-vinyl benzyl-terminated poly(ethylene oxide), p-vinyl benzyl-terminated(ethylene glycol), and mixtures thereof
One class of useful copolymerizable oligomers and macromonomers are those having a polymeric moiety with a glass transition temperature, Tg, greater than 20xc2x0 C. as described in U.S. Pat. No. 4,554,324 (Husman et al.), incorporated herein by reference. Such copolymerizable oligomers and macromonomers include, for example, ethylmethacrylate-terminated polystyrene (having a molecular weight of approximately 13,000) available as CHEMLINK 4500 from Sartomer Co., West Chester, Pa. Other useful polymerizable oligomers and macromonomers include acrylate-terminated poly(ethylene) glycols, such as acrylate-terminated poly(ethylene oxide) (having a molecular weight of 550), available as AM-90G from Shin-Nakmura Inc., Japan.
Other copolymerizable materials that may be included in the emulsion or the pressure sensitive adhesive are prepolymerized materials. These materials may be in a syrup. The prepolymerized material may be a (meth)acrylate monomer.
The other copolymerizable material described in this section typically replaces part of the (meth)acrylate monomer material. The other copolymerizable material may be used in an amount that is sufficient to modify the pressure sensitive adhesive to achieve a specific application. These materials could comprise up to 50% of the emulsion or pressure sensitive adhesive.
Polymerization via emulsion techniques generally includes the presence of at least one emulsifier, such as a surfactant or a polymeric suspending agent. These types of materials allow for the formation and stabilization of the emulsion. Without an emulsifier, droplets that later become latex particles typically cannot be formed.
Surfactants
Useful surfactants for the present invention include nonionic surfactants, anionic surfactants, cationic surfactants, and mixtures thereof Optionally, the surfactant is copolymerizable with the (meth)acrylate monomer material and the polar, free-radically copolymerizable material.
Suitable nonionic surfactants include, but are not limited to, those surfactants with molecular structures that can be formed as a condensation product of a hydrophobic aliphatic or alkyl aromatic compound with a hydrophilic alkylene oxide, such as ethylene oxide. The Hydrophilic-Lipophilic Balance (HLB) of typical nonionic surfactants is about 10 or greater and usually ranges from about 15 to about 20. The HLB of a surfactant is an expression of the balance of the size and strength of the hydrophilic (water-loving or polar) groups and the lipophilic or hydrophobic (oil-loving or non-polar) groups of the surfactant and is generally indicated by the manufacturer of the surfactant.
Commercial examples of suitable nonionic surfactants include, but are not limited to, nonylphenoxy and octylphenoxy poly(ethyleneoxy) ethanols available, for example, as the IGEPAL CA and CO series, respectively from Rhone-Poulenc, Inc., Cranberry, N.J.; C11 to C15 secondary-alcohol ethoxylates available, for example, as the TERGITOL 15-S series, including 15-S-7, 15-S-9, 15-S-12, from Union Carbide Chemicals and Plastics Co., Gary, Ind.; polyoxyethylene sorbitan fatty acid esters available, for example, as the TWEEN series of surfactants from ICI Chemicals, Wilmington, Del.; polyethylene oxide(25) oleyl ether available, for example, as SIPONIC Y-500-70 from Americal Alcolac Chemical Co., Baltimore, Md.; alkylaryl polyether alcohols available, for example, as the TRITON X series, including X-100, X-165, X-305, and X-405, from Union Carbide Chemicals and Plastics Co., Gary, Ind.
Useful anionic surfactants include, but are not limited to, those with molecular structures having (1) at least one hydrophobic moiety, such as, for example, C6 to C12 alkyl, alkylaryl, and alkenyl groups and (2) at least one anionic group, such as, for example, sulfate, sulfonate, phosphate, polyoxyethylene sulfate, polyoxyethylene sulfonate, polyoxyethylene phosphate, and the like, or the salts of such anionic groups, including, for example, the alkali metal salts, ammonium salts, tertiary amino salts, and the like.
Representative commercial examples of suitable anionic surfactants include, for example, sodium lauryl sulfate, available as TEXAPON L-100 from Henkel Inc., Wilmington, Del., or as POLYSTEP B-3 from Stepan Chemical Co, Northfield, Ill.; sodium lauryl ether sulfate, available as POLYSTEP B-12 from Stepan Chemical Co., Northfield, Ill.; ammonium lauryl sulfate, available as STANDAPOL A from Henkel Inc., Wilmington, Del.; and sodium dodecyl benzene sulfonate, available as SIPONATE DS-10 from Rhone-Poulenc, Inc., Cranberry, N.J.
Other suitable anionic surfactants include, but are not limited to, ethylenically-unsaturated copolymerizable surfactants of the formula: Rxe2x80x94Oxe2x80x94(Rxe2x80x2O)mxe2x80x94(CH2CH2O)nxe2x88x921xe2x80x94CH2CH2X. R is selected from the group consisting of C12 to C18 alkenyl, acrylyl, acrylyl alkyl, methacrylyl, methacrylyl alkyl, vinylphenyl and vinylphenylene. Rxe2x80x2O is a bivalent alkyleneoxy group derived from an epoxy compound having more than two carbon atoms such as, for example, propylene oxide or butylene oxide. In addition, m represents an integer of about 5 to about 100 and n represents an integer of about 5 to about 100. The ratio of m to n generally ranges from about 20:1 to about 1:20. The ratio of m to n will typically influence the HLB of the polymerizable surfactant. The HLB for suitable anionic copolymerizable surfactants, exclusive of the X-group, ranges from about 8 to about 18. X is an anionic group such as, for example, sulfonate, sulfate, and phosphate. In addition, the alkali metal salts, ammonium salts, and tertiary amino salts of these compounds can be used.
Examples of copolymerizable anionic surfactants include alkylene polyalkoxy sulfate available as MAZON SAM 211 from PPG Industries, Inc., Gurnee, Ill. and o-propylene-p-alkyl phenolethoxy ammonium sulfate available as HS-10 from DKS International, Inc., Japan.
Suitable cationic surfactants include, but are not limited to, quaternary ammonium salts having at least one higher molecular weight substituent and at least two or three lower molecular weight substituents linked to a common nitrogen atom. The counter ion to the ammonium cation is, for example, a halide (bromide, chloride, iodide, or fluoride), acetate, nitrite, or lower alkosulfate (e.g., methosulfate). The higher molecular weight substituent(s) of the ammonium cation are, for example, alkyl group(s), containing about 10 to about 20 carbon atoms. The lower molecular weight substituents of the ammonium cation are, for example, alkyl groups of about 1 to about 4 carbon atoms, such as methyl or ethyl. These alkyl groups are optionally substituted with hydroxy moieties. Optionally, one or more of the substituents of the ammonium cation can include an aryl moiety or be replaced by an aryl, such as benzyl or phenyl. Also among the possible lower molecular weight substituents are lower alkyls of about 1 to about 4 carbon atoms, such as methyl and ethyl, substituted by lower polyalkoxy moieties such as polyoxyethylene moieties, bearing a hydroxyl end group. These moieties fall within the general formulaxe2x80x94R(CH2CH2O)(nxe2x88x921)xe2x80x94CH2CH2OH where xe2x80x94R is the C1 to C4 alkyl group bonded to the nitrogen, and n represents an integer of about 1 to about 15. Alternatively, one or two of such lower polyalkoxy moieties having terminal hydroxyls can be directly bonded to the nitrogen.
Examples of suitable quaternary ammonium halide surfactants include, but are not limited to, trimethyl alkyl benzyl ammonium chloride, available as VARIQUAT 50MC from Witco Corp., Greenwich, Conn.; methylbis(2-hydroxyethyl)co-ammonium chloride or oleyl-ammonium chloride, available as ETHOQUAD C/12 and ETHOQUAD O/12, respectively, from Akzo Chemical Inc., Matawan, N.J.; and methyl polyoxyethylene octadecyl ammonium chloride, available as ETHOQUAD 18/25 from Akzo Chemical Inc., Matawan, N.J.
Generally, the surfactant is provided in the emulsion in an amount ranging from about 0.05 to about 8 parts by weight for 100 parts by weight of the total free-radically copolymerizable material. Typically, the surfactant is provided in an amount ranging from about 0.1 to about 3 parts by weight.
Polymeric Suspending Agents
Polymeric suspending agents can also be used in the emulsion, either alone or in combination with one or more surfactants, to stabilize the emulsion. Suitable polymeric suspending agents are those conventionally used in emulsion polymerization processes and include, for example, water-soluble organic suspending agents such as, for example, polyacrylic acid and polyvinyl alcohol.
Generally, the polymeric suspending agent is provided in the emulsion in an amount ranging from about 0.05 to about 8 parts by weight for 100 parts by weight of the total free-radically copolymerizable material. Typically, the polymeric suspending agent is provided in an amount ranging from about 0.1 to about 3 parts by weight.
Crosslinking agents can optionally be added to the emulsion composition to influence the cohesive strength and other properties of the pressure sensitive adhesive. For example, a desired additive may have been added to the emulsion that results in a reduction in the emulsion""s cohesive strength, and the crosslinking agents may be necessary to enhance the cohesive strength. The crosslinking agents are copolymerizable. Examples of copolymerizable crosslinking agents include, but are not limited to, alkyl diacrylates, alkyl triacrylates, and alkyl tetracrylates. Examples of specific crosslinking agents-include 1,2-ethylene glycol diacrylate, 1,4-butanediol diacrylate, 1,6-hexanediol diacrylate, 1,8-octanediol diacrylate, 1,12-dodecanediol diacrylate, trimethylol propane triacrylate, pentaerythritol tetraacrylate, and mixtures thereof.
Other suitable multifunctional crosslinking agents include oligomeric and polymeric multifunctional acrylates and methacrylates. Specific examples of these materials include poly(ethylene oxide) diacrylate, poly(ethylene oxide) dimethylacrylate, and difunctional urethane acrylates, such as, for example, EBECRYL 270 and EBECRYL 230 (1500 weight average molecular weight and 5000 weight average molecular weight acrylated urethanes, respectively) both available from Radcure Specialties, Atlanta, Ga.
Crosslinking agents, when used, are generally provided in the emulsion in an amount ranging from about 0.005 to about 5 parts by weight for 100 parts by weight of the total free-radically copolymerizable material. Typically, the crosslinking agents are provided in an amount ranging from about 0.01 to about 2 parts by weight.
The emulsion composition can optionally include a chain transfer agent. Chain transfer agents can be added to control the molecular weight of the resulting pressure sensitive adhesive. The chain transfer agent acts to terminate the polymerization process, causing the polymer to have a shorter chain length, and thus a lower molecular weight than it might otherwise have. In general, the more chain transfer agent added, the lower the average molecular weight of the resulting polymer. Examples of suitable chain transfer agents include, but are not limited to, organic solvents, carbon tetrabromide, alcohols, mercaptans, and mixtures thereof Specific suitable chain transfer agents include n-dodecyl mercaptan, isooctylthioglycolate, pentaerythritol tetrathioglycolate, and carbon tetrabromide.
Chain transfer agents, when used, are provided in the emulsion in an amount ranging from about 0.01 to about 5 parts by weight for 100 parts by weight of the total free-radically copolymerizable material. Typically, the chain transfer agent is provided in an amount ranging from about 0.05 to about 2 parts by weight.
In some embodiments, the combination of relatively low levels of crosslinking agent and chain transfer agent can result in high peel adhesion and shear properties. In these embodiments, the amount of crosslinking agent can be, for example, 0.01 to 0.1 parts by weight and the amount of chain transfer agent can be for example, 0.01 to 0.1 parts by weight.
Optionally, thickeners can be added to an emulsion composition, preferably after polymerization, to influence the viscosity or other properties of the resulting pressure sensitive adhesive. Adding a polymer or copolymer thickener, a polysaccharide thickener, or an inorganic thickener to the emulsion can alter the viscosity of the polymerized emulsion. Typically, increasing the viscosity of the emulsion can make it easier to coat the emulsion onto a substrate.
A polysaccharide or inorganic thickener can be used to modify the emulsion""s coating viscosity or when thicker pressure-sensitive adhesive layers are desired. Examples of suitable polysaccharide thickeners include starches such as corn starch. Suitable inorganic thickening agents include, for example, silicas such as hydrophilic silica available under the trade name CAB-O-SIL M5 from Cabot Corporation, Tuscola, Ill. and colloidal silicas available under the trade names NALCO 2327 or NALCO 1034A from Nalco Chemical Co., Naperville, Ill.
When used, the polysaccharide or inorganic thickener is provided in the emulsion in an amount ranging from about 1 to about 10 parts by weight for 100 parts by weight of the total free-radically copolymerizable material. Typically, the polysaccharide or inorganic thickener is provided in an amount ranging from about 1 to about 3 parts by weight.
Optionally, tackifying agents can be added to the emulsion composition to alter peel and shear properties of the resulting pressure sensitive adhesive. Useful tackifying agents include, for example, hydrogenated hydrocarbon resins, phenol modified terpenes, poly(t-butyl styrene), synthetic hydrocarbon resins, rosin esters, vinyl cyclohexane, and the like. Specific examples of such tackifying agents include synthetic and natural resins available as REGALREZ 1085, REGALREZ 1094, REGALREZ 6108, PICCOLYTE S-115, and FORAL 85, all from Hercules Chemical Co., Wilmington, Del.; WINGTACK PLUS, from Goodyear Tire and Rubber Company, Akron, Ohio; ESCOREZ 1310, from Exxon Chemical Co., Houston, Tex.; and ARKON P-90, from Arakawa Chemical Industries, Osaka, Japan.
When used, the tackifying agent is provided in the emulsion in an amount ranging from about 0.5 to about 30 parts by weight for 100 parts by weight of the total free-radically copolymerizable material. Typically, the tackifying agent is provided in an amount ranging from about 1 to about 15 parts by weight.
One of the major advantages of the present invention is that chemical free radical initiators are not needed to initiate the polymerization process. Subjecting the water phase of the emulsion to gamma rays generates hydrogen and hydroxyl free radicals (on the order of 1 hydrogen radical and 3 hydroxyl radical per 100 eV of absorbed energy). These free radicals are highly reactive and provide free radical initiation without the need of chemical initiators. Additionally, gamma rays cause monomer decomposition in the oil droplets, which also generates free radicals that initiate polymerization in the oil droplets.
If desired, however, chemical initiators (e.g., thermal initiators or photoinitiators) can be used, for example, for pre- or post-irradiation polymerization or crosslinking. Adding initiators can change the properties of the resulting pressure sensitive adhesive. For example, initiators may increase shear strength and decrease peel adhesion or vice versa. In addition, initiators may decrease the optical clarity of the polymer.
The emulsion of the invention can optionally contain one or more conventional additives including plasticizers, dyes, pigments, fillers, antioxidants, antiozonants, UV stabilizers, electro-conductivity agents, reflective agents, antistatic agents, inorganic materials, biocides (bactericides, fungicides), bioactive agents, pharmaceutical aids, releasable agents, cosmetic agents, rheology modifiers, and the like.
The emulsions described herein can be made with a higher content of solids (and in particular, the polymerizable material,) than is generally possible when using other batch emulsion methods (which typically have a solid content of about 20%). Advantageously, a high solids content typically provides a viscosity that makes it easy to coat the emulsion and to control the coating thickness. The high solids content can also reduce any heat or time needed to obtain a dry polymerized coating because there is a smaller amount of water used to make the emulsion. A lower solids emulsion can still be used but will typically have a lower viscosity unless a thickener is present to increase the viscosity of the polymerized emulsion.
At the relatively low dose rates typically used in the experiments described herein, there was generally no coagulation, even at 40% to 50% solids. Higher dose rates generally caused emulsions with high solids contents to coagulate. The ability to agitate or to circulate the emulsion may reduce the tendency to coagulate.
The emulsions, after polymerization by gamma irradiation, are easily coated onto substrates, e.g., backing materials, by conventional coating techniques. A wide variety of substrates can be used. The substrates can be any materials conventionally used as a tape backing, optical film, or any other flexible material. Examples of suitable substrates include substrates made of paper or wood, plastic films made using polymers such as, for example, polyethylene, polypropylene, polyurethane, polyvinyl chloride, polyester (e.g., polyethylene terephthalate), polystyrene, polycarbonates, polyphenylene oxides, polyimides, polyvinyl fluoride, polyvinylidene fluoride, polytetrafluoroethylene, cellulose acetate, and ethyl cellulose.
Substrates can also be made of fabric such as woven fabric formed from threads of synthetic or natural materials such as cotton, nylon, rayon, glass, ceramic materials, and the like, or nonwoven fabric such as air-laid webs of natural or synthetic fibers, or blends of these. Materials normally used as release liners can also be used. These include silicone-coated polymer film or paper and polyethylene-coated paper. The substrates can also be formed of metal and other inorganic materials such as, for example, aluminum foil, copper foil, tin foil, steel panels, metalized polymer films, metalized plastics, glass, and ceramic sheet materials. The substrates can be flexible or rigid, and can be occlusive or non-occlusive. The substrates can take the form of any article conventionally known to be used with pressure sensitive adhesive compositions including articles such as, for example, labels, tapes, signs, covers, marking indicia, and the like.
A variety of known methods are available for making the emulsions using the components described above. An example of one method includes mixing deionized water with the emulsifier in a homogenizing mixer. Examples of suitable mixers include the Greerco homogenizer (Model #12, Greerco Corp., Hudson, N.H. and the OMNI homogenizer (Model #17105, OMNI Corp. International, Waterbury, Conn.). The surfactant and water mixture can be initially agitated at a medium speed setting to dissolve the surfactant in the deionized water and then mixed at a high speed setting to form an emulsion.
Once the emulsifier is dissolved in the water, the other components of the emulsion (which have typically been premixed) are added to the emulsion. The emulsion is mixed under high shear conditions until small droplets are formed.
The emulsion is irradiated with gamma rays to initiate polymerization by free radical generation in response to the gamma rays impacting the emulsion. In particular, the gamma rays generate free radicals throughout the emulsion. This was done by way of, for example, Compton electrons that result from the elastic collision between a photon and a loosely bound or unbound electron. In this process, the photon energy is reduced and the electron is set in motion.
In one embodiment, the entire emulsion is positioned in proximity to a gamma ray source. Preferably the emulsion is irradiated in a substantially uniform manner by either repositioning the source or material, or agitating the material during irradiation. In another embodiment, only a portion of the emulsion is brought into proximity to the gamma ray source to generate free radicals. This portion can be returned to and mixed with the remainder of the emulsion to initiate polymerization in the remainder. In yet another embodiment, the emulsion is pumped around the gamma ray source (for example, through a tube looped around the gamma source) so that at least a portion of the emulsion is in proximity to the gamma rays at each point in time.
In general, the dose rate in a gamma irradiator is determined by the source strength at the time of irradiation and the distance from the source to the target (e.g., emulsion). Generally, suitable gamma ray sources emit gamma rays having energies of 400 keV or greater. Typically, suitable gamma ray sources emit gamma rays having energies in the range of 500 keV to 5 MeV. Examples of suitable gamma ray sources include cobalt-60 isotope (which emits photons with energies of approximately 1.17 and 1.33 MeV in nearly equal proportions) and cesium-137 isotope (which emits photons with energies of approximately 0.662 MeV). The distance from the source can be fixed or made variable by changing the position of the target or the source. The flux of gamma rays emitted from the source generally decays with (1) the square of the distance from the source and (2) duration of time as governed by the half-life of the isotope.
Once a dose rate has been established, the absorbed dose is accumulated over a period of time. During this period of time, the dose rate may vary if the source or target is in motion. For any given piece of equipment and irradiation sample location, the dosage delivered can be measured in accordance with ASTM E-1702 entitled xe2x80x9cPractice for Dosimetry in a Gamma Irradiation Facility for Radiation Processingxe2x80x9d. Specifically, all of the dosimetry reported in the examples was done per ASTM E-1275 entitled xe2x80x9cPractice for Use of a Radiochromic Film Dosimetry Systemxe2x80x9d using Far West Technologies (Goleta, Calif.) thin film dosimeters.
Dose is the total amount of energy absorbed per mass unit. Dose is commonly expressed in Megarads (Mrads) or kiloGrays (kGy). A Mrad is 10 kiloGrays. A Gray is defined as the amount of radiation required to supply 1 joule of energy per kilogram of mass. The total dose received by the emulsion depends on a number of parameters including source activity, residence time (i.e., the total time the sample is irradiated), the distance from the source, and attenuation by the intervening cross-section of materials between the source and emulsion. Dose is typically regulated by controlling residence time, distance to the source, or both.
The total dose received by the emulsion can affect the extent of polymerization and crosslinking. Generally, it is desirable to convert at least 80 wt. % of the (meth)acrylate monomer material to polymer. Preferably, at least 90 wt. % or 95 wt. % of the (meth)acrylate monomer material is converted to polymer. The dose needed for polymerization depends on a variety of factors including, for example, the materials used in the emulsion, the desired properties, the presence/absence and amount of crosslinking agent, the presence/absence and amount of chain transfer agent, the presence and amount of free radical inhibitors or free radical scavengers present, such as dissolved oxygen, and the desired properties. Generally, it was found that doses in the range of about 0.02 to 40 kGy were suitable. In particular, it was found that doses of about 0.5 to 5 kGy at the dose rate used in the examples were sufficient to obtain pressure sensitive adhesives with a range of peel adhesion and shear properties that were suitable for a wide range of applications. Total dose requirement for any given composition will vary as a function of the dose rate. As the dose rate increased, the dose requirement increased to overcome an increased level of radical termination that typically takes place.
Higher dose rates typically result in formation of low molecular weight material and highly-crosslinked polymers. Excessive crosslinking and/or the presence of low molecular weight material not incorporated in the gel may cause a pressure sensitive adhesive to have low shear and peel adhesion properties. Thus, a dose rate can be selected based on desired properties for a specified composition. The dose rate is typically in the range of 0.0001 kGy/sec and 0.01 kGy/sec.
Generally, the emulsion is purged (e.g., for two minutes or more) of air using nitrogen or another inert gas because oxygen inhibits free-radical polymerization. This purging can facilitate polymerization and high conversion in a desired period of time. However, purging is not necessary when the containers that hold the emulsion contain only a small amount of trapped air. A higher dose rate or a longer exposure time would be needed to achieve a similar degree of polymerization absent purging if a significant amount of oxygen were present.
The formation of pressure sensitive adhesives using gamma irradiation of an emulsion appears to be relatively temperature independent as long as the emulsion remains stable during polymerization.
Techniques for coating the polymerized emulsions on the substrate include any method suitable for solution coating on a substrate such as, for example, spray coating, curtain coating, casting, calendaring, knife coating, doctor blade coating, roller coating, reverse roller coating, extrusion coating, and die coating. Any desired thickness can be selected (e.g., a thickness of 25 to 50 xcexcm). Generally, the coated and polymerized emulsion is allowed to dry to evaporate the water. This can be done by air drying or drying in an oven.